From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [RFC PATCH V1 0/6] mm: add a new option MREMAP_DUP to mmrep syscall Date: Mon, 30 Dec 2013 18:23:42 -0200 Message-ID: <20131230202342.GA7973@amt.cnet> References: <1368093011-4867-1-git-send-email-wenchaolinux@gmail.com> <20130509141329.GC11497@suse.de> <518C5B5E.4010706@gmail.com> <52AFE828.3010500@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Stefan Hajnoczi , wenchao , Mel Gorman , linux-mm@kvack.org, Andrew Morton , hughd@google.com, walken@google.com, Alexander Viro , kirill.shutemov@linux.intel.com, Anthony Liguori , KVM To: Xiao Guangrong Return-path: Received: from mx1.redhat.com ([209.132.183.28]:54272 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932117Ab3L3UbB (ORCPT ); Mon, 30 Dec 2013 15:31:01 -0500 Content-Disposition: inline In-Reply-To: <52AFE828.3010500@linux.vnet.ibm.com> Sender: kvm-owner@vger.kernel.org List-ID: On Tue, Dec 17, 2013 at 01:59:04PM +0800, Xiao Guangrong wrote: >=20 > CCed KVM guys. >=20 > On 05/10/2013 01:11 PM, Stefan Hajnoczi wrote: > > On Fri, May 10, 2013 at 4:28 AM, wenchao w= rote: > >> =E4=BA=8E 2013-5-9 22:13, Mel Gorman =E5=86=99=E9=81=93: > >> > >>> On Thu, May 09, 2013 at 05:50:05PM +0800, wenchaolinux@gmail.com = wrote: > >>>> > >>>> From: Wenchao Xia > >>>> > >>>> This serial try to enable mremap syscall to cow some private = memory > >>>> region, > >>>> just like what fork() did. As a result, user space application w= ould got > >>>> a > >>>> mirror of those region, and it can be used as a snapshot for fur= ther > >>>> processing. > >>>> > >>> > >>> What not just fork()? Even if the application was threaded it sho= uld be > >>> managable to handle fork just for processing the private memory r= egion > >>> in question. I'm having trouble figuring out what sort of applica= tion > >>> would require an interface like this. > >>> > >> It have some troubles: parent - child communication, sometimes > >> page copy. > >> I'd like to snapshot qemu guest's RAM, currently solution is: > >> 1) fork() > >> 2) pipe guest RAM data from child to parent. > >> 3) parent write down the contents. > >> > >> To avoid complex communication for data control, and file conten= t > >> protecting, So let parent instead of child handling the data with > >> a pipe, but this brings additional copy(). I think an explicit API > >> cow mapping an memory region inside one process, could avoid it, > >> and faster and cow less pages, also make user space code nicer. > >=20 > > A new Linux-specific API is not portable and not available on exist= ing > > hosts. Since QEMU supports non-Linux host operating systems the > > fork() approach is preferable. > >=20 > > If you're worried about the memory copy - which should be benchmark= ed > > - then vmsplice(2) can be used in the child process and splice(2) c= an > > be used in the parent. It probably doesn't help though since QEMU > > scans RAM pages to find all-zero pages before sending them over the > > socket, and at that point the memory copy might not make much > > difference. > >=20 > > Perhaps other applications can use this new flag better, but for QE= MU > > I think fork()'s portability is more important than the convenience= of > > accessing the CoW pages in the same process. >=20 > Yup, I agree with you that the new syscall sometimes is not a good so= lution. >=20 > Currently, we're working on live-update[1] that will be enabled on Qe= mu firstly, > this feature let the guest run on the new Qemu binary smoothly withou= t > restart, it's good for us to do security-update. >=20 > In this case, we need to move the guest memory on old qemu instance t= o the > new one, fork() can not help because we need to exec() a new instance= , after > that all memory mapping will be destroyed. >=20 > We tried to enable SPLICE_F_MOVE[2] for vmsplice() to move the memory= without > memory-copy but the performance isn't so good as we expected: it's du= e to > some limitations: the page-size, lock, message-size limitation on pip= e, etc. > Of course, we will continue to improve this, but wenchao's patch seem= s a new > direction for us. >=20 > To coordinate with your fork() approach, maybe we can introduce a new= flag > for VMA, something like: VM_KEEP_ONEXEC, to tell exec() to do not des= troy > this VMA. How about this or you guy have new idea? Really appreciate = for your > suggestion. >=20 > [1] http://marc.info/?l=3Dqemu-devel&m=3D138597598700844&w=3D2 > [2] https://lkml.org/lkml/2013/10/25/285 Hi, What is the purpose of snapshotting guest RAM here, in the context of local migration?